This is sockets/scm_rights_recv.c, an example program file from the book, The Linux Programming Interface.

This file is not printed in the book; it is a supplementary file for Chapter 61.

The source code file is copyright 2010, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

  Cover of The Linux Programming Interface

Function list (Bold in this list means a function is not static)

/* scm_rights_recv.c

   Used in conjunction with scm_rights_send.c to demonstrate passing of
   file descriptors via a UNIX domain socket.

   This program receives a file descriptor sent to a UNIX domain socket.

   Usage is as shown in the usageErr() call below.

   File descriptors can exchanged over stream or datagram sockets. This
   program uses stream sockets by default; the "-d" command-line option
   specifies that datagram sockets should be used instead.

   This program is Linux-specific.
#include "scm_rights.h"

#define BUF_SIZE 100
main(int argc, char *argv[])
    struct msghdr msgh;
    struct iovec iov;
    int data, lfd, sfd, fd, opt;
    ssize_t nr;
    Boolean useDatagramSocket;

    /* Allocate a char array of suitable size to hold the ancillary data.
       However, since this buffer is in reality a 'struct cmsghdr', use a
       union to ensure that it is aligned as required for that structure. */
    union {
        struct cmsghdr cmh;
        char   control[CMSG_SPACE(sizeof(int))];
                        /* Space large enough to hold an 'int' */
    } control_un;
    struct cmsghdr *cmhp;

    /* Parse command-line arguments */

    useDatagramSocket = FALSE;

    while ((opt = getopt(argc, argv, "d")) != -1) {
        switch (opt) {
        case 'd':
            useDatagramSocket = TRUE;

            usageErr("%s [-d]\n"
                     "        -d    use datagram socket\n", argv[0]);

    /* Create socket bound to well-known address */

    if (remove(SOCK_PATH) == -1 && errno != ENOENT)
        errExit("remove-%s", SOCK_PATH);

    if (useDatagramSocket) {
        fprintf(stderr, "Receiving via datagram socket\n");
        sfd = unixBind(SOCK_PATH, SOCK_DGRAM);
        if (sfd == -1)

    } else {
        fprintf(stderr, "Receiving via stream socket\n");
        lfd = unixListen(SOCK_PATH, 5);
        if (lfd == -1)

        sfd = accept(lfd, NULL, NULL);
        if (sfd == -1)

    /* Set 'control_un' to describe ancillary data that we want to receive */

    control_un.cmh.cmsg_len = CMSG_LEN(sizeof(int));
    control_un.cmh.cmsg_level = SOL_SOCKET;
    control_un.cmh.cmsg_type = SCM_RIGHTS;

    /* Set 'msgh' fields to describe 'control_un' */

    msgh.msg_control = control_un.control;
    msgh.msg_controllen = sizeof(control_un.control);

    /* Set fields of 'msgh' to point to buffer used to receive (real)
       data read by recvmsg() */

    msgh.msg_iov = &iov;
    msgh.msg_iovlen = 1;
    iov.iov_base = &data;
    iov.iov_len = sizeof(int);

    msgh.msg_name = NULL;               /* We don't need address of peer */
    msgh.msg_namelen = 0;

    /* Receive real plus ancillary data */

    nr = recvmsg(sfd, &msgh, 0);
    if (nr == -1)
    fprintf(stderr, "recvmsg() returned %ld\n", (long) nr);

    if (nr > 0)
        fprintf(stderr, "Received data = %d\n", data);

    /* Get the received file descriptor (which is typically a different
       file descriptor number than was used in the sending process) */

    cmhp = CMSG_FIRSTHDR(&msgh);
    if (cmhp == NULL || cmhp->cmsg_len != CMSG_LEN(sizeof(int)))
        fatal("bad cmsg header / message length");
    if (cmhp->cmsg_level != SOL_SOCKET)
        fatal("cmsg_level != SOL_SOCKET");
    if (cmhp->cmsg_type != SCM_RIGHTS)
        fatal("cmsg_type != SCM_RIGHTS");

    fd = *((int *) CMSG_DATA(cmhp));
    fprintf(stderr, "Received fd=%d\n", fd);

    /* Having obtained the file descriptor, read the file's contents and
       print them on standard output */

    for (;;) {
        char buf[BUF_SIZE];
        ssize_t numRead;

        numRead = read(fd, buf, BUF_SIZE);
        if (numRead == -1)

        if (numRead == 0)

        write(STDOUT_FILENO, buf, numRead);



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